This invention relates to a two-component membrane for separating at least one liquid from a mixture of plural liquids by pervaporation; to a process for preparing the membrane in one of plural arrays in a module for effecting a desired separation; and to the module which contains plural arrays of the membrane. This pervaporation process is a membrane process specifically for the removal and concentration of an organic compound from an aqueous phase in which it the organic compound is present in a minor proportion by weight. In a specific application, pervaporation is used to separate at least one volatile organic compound ("VOC" for brevity) from water in which the VOC is present in less than 1 percent by weight.
More particularly, the invention relates to a pervaporation membrane consisting essentially of a microporous support which is impregnated and fully penetrated from the support's interior surface to its exterior surface, by a permselective essentially non-porous, solid, non-removable organophilic polymer, permeable to the target component. Because the organophilic polymer fills the pores and channels of the support, the pervaporation membrane is at least as thick as the support within it; and because the pores and channels are through-impregnated from surface to surface, the support is said to be "filled" with the polymer.
By "non-removable" we mean that the polymer cannot be removed from the support without destroying the microporous structure of the support. The non-porous organophilic polymer is referred to herein as a "filling" which is contained within a microporous support. Because the support is to be filled its pores and channels are permeable to the organophilic polymer. The term "coating" is used herein to denote the filling in combination with interior and exterior "skins" of the organophilic polymer, as explained in greater detail herebelow. A preferred organophilic polymer is an elastomer, preferably a silicone polymer, and when the support is impregnated with an elastomer, the support is referred to as being elastomer-permeable.
The microporous support, by itself, has essentially no membranous function with respect to the target component to be separated by pervaporation. As will be more fully explained herebelow, the function of the microporous support is to reinforce the coating which essentially completely impregnates the support. Therefore the impregnated support has essentially only a pervaporation function, namely, to separate the desired target component from a mixture containing it. The microporous support in our pervaporation membrane is therefore referred to herein as a "non-membranous microporous support" (or `NMMS` for brevity). In addition to impregnating walls of the NMMS, the coating may cover the exterior and/or interior surfaces of the walls, forming exterior and interior "skins", respectively, but the impregnated support is at least about 20 .mu.m (micrometers or microns) thick, this being the thinnest cross-section of a wall of a microporous support believed to be now available.
This invention also relates to a pervaporation process for separating one or more desired liquids (target components) from a mixture containing them, based on the difference in the rate of permeation of the desired liquid(s) to be separated, from that of at least one other liquid in the mixture. Separation therefore provides a permeate having a different concentration of the desired liquid(s) compared with the concentration of the at least one other liquid in the mixture. Such a difference in permeation rate occurs when there is a chemical potential gradient for the target component between two zones, the higher potential being present in the feed zone through which the mixture flows, and the lower potential being present in the permeate zone.
The term "microporous support" is used, though the NMMS has relatively large pores through which a relatively viscous elastomer can flow under less than 790 kPa absolute (100 psig) pressure without damaging the integrity of the microporous support.
Still more particularly, this invention relates to the manufacture of a pervaporation module for the removal of a trace amount of a target organic compound, typically a VOC from an aqueous phase, or a VOC from an oil-water emulsion. The removal is effected by using a multiplicity of hollow two-component membranes (referred to as "hollow fibers") held in arrays housed in a module constructed to define a feed zone and a permeate zone. By "trace amount" we refer to a concentration which is typically less than 1% by weight.
In a typical pervaporation module, liquid feed flowing through the feed zone is in contact with one side of the two-component membrane consisting essentially of an essentially non-porous or "dense" organophilic polymer supported on the NMMS (non-membranous microporous support). The NMMS is necessary only because known organophilic polymers having desirable flux-differentiation characteristics lack sufficient strength as a self-supporting layer. By "flux" we refer to the permeation rate of the permeate per unit surface area. In the thickness suitable for use as a coating in this invention, a suitable organophilic polymer is non-self-supporting, and it is only such a non-self-supporting coating which is of interest as a component of a two-component hollow fiber.
In such a hollow fiber, the NMMS per se, may be a relatively insignificant barrier to the passage of both water (or organic liquid) and target VOC. When the NMMS is filled with organophilic polymer passage of water is restricted to that which diffuses through the coating. Therefore any pores in the coating, particularly such pores referred to as "pin holes" typically found in thin layers of polymer deposited on the surface of a typical porous permselective membrane, vitiate the performance of a pervaporation module. In an effective pervaporation module, both water (or organic liquid) and VOC are absorbed into the coating and diffuse through it to the permeate zone from which the VOC and some water (or organic liquid) are drawn off by vacuum.
The pores of prior art porous supports served only to allow the polymer to form a `compact membrane` to bridge across the pores, as for example clearly illustrated in U.S. Pat. No. 4,230,463 to Henis et al (class 55/subclass 16). However, as stated therein, "though such compact membranes are fairly selective, one of their main disadvantages is low permeate flux due to the relatively large thickness associated with the membranes." (see col 2, lines 36-39). Yet we have found such a compact membrane to be ideal, provided it is thick enough to contain a NMMS. Further, unlike the '463 membrane, the separation properties of our two-component membrane are principally determined by the coating as opposed to the NMMS. We do not use a porous separation membrane such as is used in the '463 multicomponent membrane (see col 6, lines 33-37). Essentially the same emphasis on a "thin" coating over the outer surface of a membrane composite is taught in U.S. Pat. No. 3,980,456 to Browall.
The direction of the art towards thinner coatings, rather than thicker ones, over a porous support is also evidenced in U.S. Pat. No. 4,978,451 to Taylor. He used a porous support but made sure that the layer coating the support was ultrathin (as thin as 1 .mu.m) yet with "sufficient structural integrity while being significantly more efficient as a diffusion membrane when compared to a thicker unsupported membrane of the same type". (see col 1, lines 44-49). This choice of a very thin rather than a thick membrane, was made by Blume et al in U.S. Pat. No. 4,931,181.
To concentrate the VOC greatly, compared with its concentration in the feed, the coating of our two-component membrane is distinguished by having a relatively high flux for the VOC and a relatively low flux for water. The concentration of the VOC in the permeate is by a factor of at least 10, and preferably by a factor of 50, 100, 1000 or 10,000. The VOC is therefore referred to herein as being concentrated by orders of magnitude relative to its concentration in the feed. When the desired concentration of VOC is achieved in the permeate zone, separation of the VOC from the water is relatively easy, if such separation is desired to be made.
For the most part in this specification, for convenience, and to facilitate understanding the thrust of the invention, we shall refer to a separation of a VOC from water, it being understood that the VOC could also be recovered from an organic liquid, or mixture of water and organic liquid, in an analogous manner.
The general theoretical technical basis and considerations relating to the foregoing technology are set forth in a paper titled "A Technico-Economical Evaluation of Pervaporation for Water Treatment" presented by Pierre Cote and Chris Lipski at the Fourth International Conference on Pervaporation Processes in the Chemical Industry, held in Fort Lauderdale, Fla. from Dec. 3-7 1989, and, in an article titled "The Use of Pervaporation for the Removal of Organic Contaminants from Water" by Chris Lipski and Pierre Cote in Environmental Progress Vol 9, No. 4, pg 254-261, the disclosures of which are incorporated by reference thereto as if fully set forth herein. The thrust of the disclosures relating to improving the performance of a pervaporation membrane pointed out that a relatively thick discriminating layer from 30 .mu.m to 100 .mu.m, is more effective than a thinner layer because the thicker layer decreases the flux of water through it far more than the thinner layer, without significantly reducing the flux of the VOC. The theoretically derived conclusions were experimentally confirmed by using homogeneous silicone fibers having thick walls. But such fibers have inadequate strength for practical use. The problem of providing thick-walled hollow fibers which functioned as thick pervaporation membranes with adequate strength, was solved by our invention.
Our discovery that thick permselective membranes for use in pervaporation were more effective than thin membranes was referenced by Henry Nijhuis in his Doctoral thesis titled "Removal of Trace Organics From Water by Pervaporation", published in October 1990, relevant portions relating to the formation, use and effectiveness of a thick film are incorporated by reference thereto as if fully set forth herein. Nijhuis confirmed that water fluxes, which are independent of the concentration of organics in the aqueous solution, appear to be inversely proportional to the effective thickness of the membrane. In a cost analysis for pervaporation with silicone rubber membranes the total permeate flux (mainly water) through 10 .mu.m membranes was estimated at 130 g/m.sup.2 at 99% recovery. The condensation costs for the permeate handling are reduced from Dfl 1,040,000 (US$ 520,000) to Dfl 80,000 (page 154) using a less permeable yet still quite thick ethylene propylene terpolymer.
Rather than switching membranes (because ethylene propylene terpolymer is difficult to work with) Nijhuis confirmed (page 44) that a 100 .mu.m thick silicone membrane could be substituted for the 10 .mu.m silicone membrane with no significant reduction in the organic removal but it would reduce the water handling requirement by a factor of 10 (page 40) and reduce the condensation costs to Dfl 104,000.
It follows that an ideal module for separation of a VOC from polluted water comprises a multiplicity of hollow fibers housed in a hydrodynamically efficient module, each fiber having a thick coating of organophilic polymer, at least 20 .mu.m thick, preferably from 30.mu.m-200 .mu.m thick, having suitably disposed therewithin, the NMMS. The polymer is chosen because of its unique affinity for the VOC to be removed from the feed zone, and the polymer's propensity to release the VOC in the permeate zone where the partial pressure of the VOC is much less than in the feed zone at normal operating temperature in the range from about 20.degree. C. to 60.degree. C. At higher operating temperatures than normal, namely 60.degree.-150.degree. C., thicker membranes up to 500 .mu.m thick may be used.
The problem is that there is no known method for controllably depositing a thick coating which not only fills a suitably porous wall of a support but also coats both, its exterior and interior surfaces, particularly when the wall is that of a microporous hollow fiber. A conventional coating which is deposited by plasma coating is found to be less than 2 .mu.m thick, and repeating to build up the thickness of the coating is neither practical nor economical.
Another problem is that, once coated with a desirable coating, a hollow fiber is difficult to incorporate into one or more arrays of hollow fibers, which arrays are to be held in fluid-tight relationship in a module, because of their polymeric surfaces. Typically such arrays are potted with a synthetic resinous material chosen to tightly adhere to the surface of the coating, but the adhesion is generally insufficient to provide a relatively permanent fluid-tight bond unless the physical and chemical properties of the organophilic polymer match those of the potting resin.
Since silicone rubber is the organophilic polymer of choice it will now be apparent why it is difficult to seal a multicomponent membrane by potting it. Silicone rubbers are known to be highly resistant to being adhesively secured to synthetic resinous materials which have been found suitable for potting compounds.